LTC3770EUH#TR Linear Technology, LTC3770EUH#TR Datasheet - Page 15

• Manufacturer Part Number: LTC3770EUH#TR
• Manufacturer: Linear Technology

Specifications of LTC3770EUH#TR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS INFORMATION
Top MOSFET Driver Supply (C
An external bootstrap capacitor C
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V
to approximately V
to store about 100 times the gate charge required by the
top MOSFET. In most applications 0.1μF to 0.47μF, X5R
or X7R dielectric capacitor is adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when
inductor current reverses. The load current at which current
reverses and discontinuous operation begins depends on
the amplitude of the inductor ripple current and will vary
with changes in V
threshold forces continuous synchronous operation,
allowing current to reverse at light loads and maintaining
high frequency operation. To prevent forcing current back
into the main power supply, potentially boosting the input
supply to a dangerous voltage level, forced continuous
mode of operation is disabled when the TRACK/SS voltage
is 20% below the reference voltage during soft-start or
tracking up. Forced continuous mode of operation is also
disabled when the TRACK/SS voltage is below 0.1V during
tracking down operation. During these two periods, the
PGOOD signal is forced low.
R4
R3
FCB
SGND
LTC3770
Figure 5. Secondary Output Loop
PGND
IN
IN
SW
V
BG
TG
IN
. Tying the FCB pin below the 0.6V
+ INTV
CC
+
•
IN
. The boost capacitor needs
B
1:N
B
, D
T1
and the BOOST pin rises
C
V
1N4148
connected to the BOOST
IN
IN
B
•
)
+
+
B
C
1μF
C
from INTV
OUT2
OUT
3770 F05
V
V
OUT2
OUT1
CC
In addition to providing a logic input to forced continuous
operation, the FCB pin provides a mean to maintain a
fl yback winding output when the primary is operating
in discontinuous mode. The secondary output V
normally set as shown in Figure 5 by the turns ratio N
of the transformer. However, if the controller goes into
discontinuous mode and halts switching due to a light
primary load current, then V
resistor divider from V
voltage V
forced until V
Fault Conditions: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In the LTC3770, the maximum sense voltage is controlled
by the voltage on the V
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
The current limit value should be checked to ensure that
I
limit generally occurs with the largest V
ambient temperature, conditions that cause the largest
power loss in the converter. Note that it is important to
check for self-consistency between the assumed MOSFET
junction temperature and the resulting value of I
heats the MOSFET switches.
Caution should be used when setting the current limit
based upon the R
current limit is determined by the minimum MOSFET
on-resistance. Data sheets typically specify nominal
and maximum values for R
A reasonable assumption is that the minimum R
lies the same percentage below the typical value as the
maximum lies above it. Consult the MOSFET manufacturer
for further guidelines.
LIMIT(MIN)
I
V
LIMIT
OUT MIN
2
=
(
OUT2(MIN)
R
> I
V
DS ON
SNS MAX
)
OUT2
OUT(MAX)
=
(
0 6
(
.
DS(ON)
)
has risen above its minimum.
below which continuous operation is
ρ
V
T
)
⎛
⎝ ⎜
RNG
OUT2
1
+
. The minimum value of current
+
of the MOSFETs. The maximum
2
1
pin. With valley current control,
R
R
to the FCB pin sets a minimum
Δ
4
3
DS(ON)
I
OUT2
L
⎞
⎠ ⎟
will droop. An external
, but not a minimum.
LTC3770
IN
at the highest
LIMIT
15
OUT2
DS(ON)
which
3770fc
is